The present invention relates to magnetically-actuated proximity switch systems for detecting and monitoring access by the opening or closing of panels, windows, doors or the like and as part of an electrically-monitored physical security system or machinery control system.
Magnetically-actuated proximity switches are often used in physical security systems and in machinery control systems and typically employ one or more reed switches in conjunction with one or more permanent actuating magnets. Examples of magnetic switches are shown, for example, in U.S. Pat. Nos. 4,339,747 (Maybee); 5,057,807 (Longly et at.); 5,128,641 (Krizman et at.); 5,233,322 (Posey); 5,293,523 (Posey); and 3,305,805 (Tann). (These and all other patents and publications mentioned anywhere in this disclosure are hereby incorporated by reference in their entireties.) The magnetically-actuated reed switch is typically mounted in or about the frame surrounding a doorway, window or access panel and has conductors leading out from it to the security or machinery monitoring control unit. The door or cover can be a safety guard on an industrial machine (such as a hydraulic press) with a safety interlock switch which detects whether the guard is closed and the machine can thereby be safely operated. The switch also has one or more permanent actuating magnets mounted in or about the door, window or access panel itself in predetermined positions so that movement away from or to close proximity of the predetermined position to the reed switch actuates the reed switch. This produces a signal received by the security or machinery monitoring control unit.
Previously, biasing magnets have been placed close to the reed switch so that the permanent biasing magnets field is strongly coupled to the reed switch thereby increasing the sensitivity of the reed to the external magnetic field(s). This permits the use of smaller and less expensive actuating magnets. Various means for making this increased sensitivity adjustable are known, and one example is by varying the position of the permanent biasing magnet along an imaginary axis parallel to the axis of polarity of the permanent biasing magnet. This is shown by U.S. Pat. No. 4,213,110 (Holce) ('110).
Some of the biasing means in the prior art are used in combinations to make high security switches, sometimes known as "balanced" switches, so that they are resistant to manipulation or deception by the use of foreign magnetic fields. To explain, with a standard magnetic switch mounted on a door, for example, it is easy for an intruder to affix an external magnet near the switch. He can then open the door without the switch changing state and therefore without activating the building alarm system. If a powerful external magnet is employed, it can even be mounted on the opposite side of the door frame and still defeat the switch. As an example of a solution to this problem, the '110 patent shows a means for biasing a reed switch and is used in combination to produce a balanced high-security switch, as shown by U.S. Pat. No. 4,210,889 (Holce) ('889). The adjusting mechanisms in the '889 patent are made during manufacture thereof and therefore do not require any screw adjustment mechanism.
The factory adjustments of the prior art switches required during their manufacture result from the material and physical tolerances being the same order of magnitude as the mechanism's operational parameters. Consequently, each switch must be adjusted during the manufacturing process to compensate for these variations. Quality control of the component properties needed to eliminate these adjustments is not economically profitable since dimensional tolerances of the components would need to be very small. Additionally, the magnetic properties vary such that the components would have to be screened.
It is further known in the art that small permanent biasing magnets can be strongly coupled individually to respective reed switches and disposed in predetermined locations so that in combination a balanced high security switch is formed. However, it is deskable that these individual permanent biasing magnets not interact. As arranged the magnetic fields of the individual bias magnets tend to cancel each other's respective magnetic fields. Thus, in the prior art if the biasing magnets are too close together the fields tended to cancel out. Furthermore, the prior magnetically-actuated proximity switches of the balanced type require many complicated parts including extra magnetic wires and bending the ends of the reeds. They require time consuming factory or field adjustments. They are more sensitive by design than a bare reed to extraneous electromagnetic fields and other electromagnetic devices such that false triggerings may occur. Furthermore, they are highly susceptible, whether singly or in combination, to demagnetization or random magnetic pole reorientation by high coercive permanent magnets brought into close proximity to their biased reed switch.
Examples of other known high security switches are shown in U.S. Pat. Nos. 2,912,540 (Sawicki); 3,974,469 (Nicholls); 4,210,889 (Holce); 4,945,340 (Brill); 4,544,903 (Grant); and 5,233,323 (Burkett et at.) A further example of a prior art high security switch is the "MSS-100-17 High Security BMS Contacts" available from Flair Electronics, Inc. of Glendora, Calif. Another example is the powered high security switch available from Sentrol, Inc. of Portland, Oreg., called the "2900 Series High Security Magnet Contact" system, and as described in the "Sentrol Security and Life Safety Sensor News, " Vol. 1, No. 2, Winter 1994, and the "2700 Series High Security Anodized Alloy Housing With Armor Cable" system. The Sentrol systems apparently have little or no control over the actuation range. To obtain a large actuation distance the Sentrol systems would need to use a very large actuator magnet since the static portions of their switches apparently are not adjustable.
A still farther example is the ADM-30/31 high security switch available from AMSECO of Carson, Calif., and as described in its two page literature entitled "ADM-30/31 Anti-Defeat High Security Contacts." It is a simple and inexpensive design using two biased reeds and three magnets in the actuator. Although the device can be termed "defeat resistant" because of the multiple reed/multiple magnet construction, it can be relatively easily actuated by a single magnet placed at either end thereof. Thus, it might more appropriately be termed a "medium" security switch. An unusual feature of the AMSECO design is that the actuator can activate the reeds in a 360 degree circle around the module holding the reeds. As will be made clearer later, this makes it easy to defeat the AMSECO switch by employing a second AMSEGO actuator.
Inasmuch as the reason for development of high security balanced-type switches is to oppose as much as possible any attempt to defeat the switch by suppressing its alarm signal while the door or other barrier is opened, it is important to consider the security shortcomings of existing designs. While many of them are quite effective at avoiding defeat from a single external magnet, all of them are subject to defeat by someone who has obtained a second actuator. The second actuator is merely placed at the same distance from the switch module as the standard actuator but at a different angle and the switch is defeated.
Another method of defeating existing switches is simply by physically removing the device from its substrate. The switch module and actuator are then maintained in their physical relationship to each other, so no alarm signal is sent. However, since they have been removed from the door, it may be opened. This problem has been partially addressed in some designs, by creating a separate alarm signal if the switch module is dismounted through a tamper detect means, but several weaknesses remain as discussed now. Some prior tamper detect means on the switch module have been obvious to an intruder and no tamper detect means has ever been applied to the actuator. This is because the actuator mounts on a door, window or moving panel to which is prohibitively costly to run wires. All wiring is to the switch module which mounts on a fixed frame to which wires can be easily run. Therefore existing designs can be defeated by removing the actuator from the door while maintaining its distance from the switch module and then rotating it to a position with respect to the switch module that allows the door to be opened without the creation of an alarm signal.
An example of a prior art tamper switch arrangement which is available from Sentrol, Inc. includes a reed switch located on the bottom of the Sentrol switch module (the mounting surface) which is isolated by a steel plate from the rest of the switch module and a plastic plaque in the center of which is mounted a small magnet. The plaque is mounted on the door and the switch assembly is mounted on top. If the switch is ripped off of the plaque, the reed switch in the bottom of the switch module is separated from the magnet in the plaque and an alarm is actuated. However, no alarm results if the switch and plaque are both ripped off. Thus, a prior art solution has been to provide attachment screws in the plaque. Then if the switch is tom off, the screws which are relatively inaccessible keep the plaque in place. This can be defeated though by the intruder running a saw through the plaque. Additionally, the plaque is about one-eighth inch thick and it can therefore be readily observed that the switch module includes a defense against tampering.
Another example of a prior art technique for tamper proofing a high security switch module is found in the Flair Electronics product previously mentioned. The switch module of that product includes a removable cover. The installer removes the cover and then mounts the switch module by driving screws through the base of the switch module into the substrate. When the cover is replaced, the screw heads are afforded some protection against removal. A spring-loaded mechanical switch is located within the switch module, and it triggers an alarm signal if the switch module cover is subsequently removed by someone seeking to access the screw heads to remove the screws. This has an advantage over the Sentrol design in that the presence of a tamper feature is not noticeable but the tamper alarm will not sound if the switch module is removed completely from the substrate through the use of a saw, for example, which is the same weakness found in the Sentrol design.
Accordingly, there is a need for an improved magnetically-actuated, high security balanced-type proximity switch system which is substantially more difficult to actuate with an external magnetic field, which particularly includes effective defenses against defeat by a second actuator, which includes effective defenses against defeat by removal of either the switch module or actuator from their respective substrates, which maintains sensitivity to permanent actuating magnetic fields, which does not require extensive manufacturing or field adjustments, which is not susceptible to demagnetization by external high-coercivity permanent magnets, which has fewer parts, which has an adjustable actuation range, and which performs identically on steel doors as on wood or aluminum doors.